Echo in a communication system, is commonly characterized as the return of a part of a transmitted signal from an end user back to the originator of the transmitted signal after a delay period. As is known in the art, a near end user transmits an uplink signal to a far end user. Conversely, the near end user receives a downlink signal from the far end user. For example, echo at the near end occurs when the near end user originates an uplink signal on an uplink path, and a part of the transmitted signal is reflected at the far end as an echo signal on a downlink path back to the near end. Echo at the far end occurs when the far end user originates a downlink signal on the downlink path, and a part of the transmitted signal is reflected at the near end as an echo signal on the uplink path back to the far end. The reflection of the transmitted signal may occur due to a number of reasons, such as an impedance mismatch in a four/two wire hybrid at the far end or feedback due to acoustic coupling in a telephone, wireless device or hands-free speaker phone. An echo signal corresponding to the delayed transmitted signal is perceived as annoying to the near end user and, in some cases, can result in an unstable condition known as “howling.”
Echo cancellers are desirable at any echo generating source at both the near end and at the far end in an attempt to eliminate or reduce the transmission of echo signals. Echo cancellers may be employed in wireless devices, such as personal data assistants (PDAs), cellular phones, two-way radios, car-kits for cellular telephones, car phones and other suitable devices that can move throughout a geographic area Additionally, echo cancellers may be employed in wireline devices, such as hands-free speaker phones, video and audio conference phones, data and fax modems, and telephones otherwise commonly referred to in the telecommunications industry as plain old telephone system (POTS) devices. Except consumer devices, echo cancellers may be employed in infrastructure devices (e.g. gateways) of communication networks like voice over packet networks and satellite networks. Hands-free speaker phones typically include a microphone to produce the uplink signal, a speaker to acoustically produce the downlink signal, an echo canceller to cancel the echo signal and a telephone circuit.
Turning now to FIG. 1, there is shown an exemplary Echo Canceller (EC) circuit. Wherein, the original signal sent from the far-end is denoted by Rin (100). Rout (120) is the post-processed signal by the Echo-Canceller-Receiving-Direction (EC-RD) (110) unit After the signal Rout is received at the near-end, a signal Sin (130) is transmitted from the near-end. The signal Sin consists of a superposition of the (1) echo of Rout that is reflected back from near-end echo sources and (2) near-end talker signal. Echo-Canceller-Sending-Direction (EC-SD) (140) receives two signals, as shown in FIG. 1, the first input signal is Rout and the second input signal is Sin. EC-SD removes the echo replica of Rout from Sin, as a result, the generated output of the EC-SD, denoted by Sout (150), is a residual signal without echo.
The performance of EC's decreases when transmitting Narrow-Band signals. Narrow-Band signals may cause the EC to adapt in such a way that will worsen its performance during subsequent transmission.
The majority of EC's disable their adaptation process during transmission of Narrow-Band signals. This action may result in severe errors, which may lead to undesirable outcomes. An example of such outcome happens during the process of “handshake” in a telephone-call or fax-call. If the adaptation process is disabled during the handshake it is likely that the handshake will not succeed.
An alternative action would be to allow the EC to adapt to the Narrow-Band signal, however the cost would be of poor performance during subsequent signals due to the fact that it is difficult and time consuming to re-adapt EC if it is already diverged due to a Narrow-Band signal.
There is a need in the field of communication for improved methods and circuits for echo cancellation during successful transmission of narrow-band signals without worsening transmission of any other type of signals.